An engine-driven working machine with a muffler, such as a chain saw, a bush cutter, and a hedge trimmer, is known. A catalyst for purifying exhaust gas of an internal combustion engine is contained in the muffler (see, for example, Patent Publication 1).
A working machine has an engine, an actuating part (for example, a chain with cutting edges in a chain saw), and a centrifugal clutch disposed between the engine and the actuating part. The centrifugal clutch is configured to connect the engine with the actuating part when a rotational speed of the engine is higher than a predetermined clutch-in rotational speed so that rotations of the engine are transmitted to the actuating part.
The engine of the working machine is designed so that when a throttle valve of a carburetor provided in the engine is in a fully-closed position, the engine rotates stably at a rotational speed which is lower than the clutch-in rotational speed. This state is referred to as an idling state (normal idling state).
When the engine is started, especially in an insufficient warming-up state, in order to stabilize starting and running operations of the engine, the engine is generally started with the throttle valve of the carburetor in a half-opened position to increase an amount of air supplied to the engine and runs while the throttle valve is maintained in the half-opened position. This state is referred to as a fast idling state.
Generally when the engine is started, a brake for the actuating part is operated in order to prevent the actuating part from being actuated unintentionally. In the fast idling state, since the rotational speed of the engine may become higher than the clutch-in rotational speed due to the throttle valve in the half-opened state, the brake is preferably used for safety. However, if the engine is started in the fast idling state without using the brake, the actuating part could be actuated at the starting. In order to enable the starting operation to be in safe even in this situation, the engine is provided with a controller which can operate in a rotational speed limitation mode in which the rotational speed of the engine is prevented from becoming higher than a predetermined limitation rotational speed after the starting operation. When the limitation rotational speed is set to a rotational speed which is equal to or lower than the clutch-in rotational speed, the centrifugal clutch does not connect the engine with the actuating pat, so that, for example, the cutting edges of the chain saw are not rotated. When the limitation rotational speed is set to a rotational speed which is a little higher than the clutch-in rotational speed, for example, the cutting edges of the chain saw may be rotated at a very low rotational speed, but this rotation can be stopped by the brake.
The controller operates in the rotational speed limitation mode from the starting of the engine. Concretely, the controller detects the rotational speed of the engine, and when excess of the rotational speed of the engine over the limitation rotational speed is expected (when the rotational speed of the engine exceeds a predetermined rotational speed which is lower than the limitation rotational speed), the controller prevents the rotational speed of the engine from becoming higher than the limitation rotational speed by performing a misfiring cycle which makes an ignition device of the engine inoperative. Additionally by retarding an ignition timing of the ignition device, the controller may prevent the rotational speed of the engine from becoming higher than the limitation rotational speed.
On the other hand, when the operation of the controller is maintained in the rotational speed limitation mode, even if the throttle valve is moved to a fully-opened position, the rotational speed of the engine is prevented from becoming higher than the limitation rotational speed. As a result, the centrifugal clutch is not operated, and the rotations of the engine may not be transmitted to the actuating part or the cutting edges may be rotated only at a very low speed. Therefore, in order to operate the working machine, it is necessary to cancel the rotational speed limitation mode at an appropriate timing (see, for example, Japanese Patent Publication 2). For example, when the throttle lever of the engine is returned after the throttle lever is once moved to the fully-opened state, the rotational speed limitation mode is cancelled so that the working machine becomes in the idling state.